The present invention relates to automotive test equipment and, in particular, to a hand-held ignition voltage tester for detecting a secondary voltage in a spark plug wire.
The majority of automobiles manufactured today use distributorless ignition systems similar to the systems that have been used on multi-cylinder motorcycle engines. Distributorless ignition systems use multiple ignition coils, with each coil having two secondary outputs connected in a series with one another. The two secondary outputs are connected to the spark plugs of a pair of companion cylinders, and both spark plugs fire simultaneously but with opposite polarities. A four cylinder engine uses two ignition coils and a six cylinder engine uses three ignition coils. In a four stroke engine, one spark plug fires during the compression stroke and the other spark plug fires during the exhaust stroke. Therefore, each spark plug fires twice every engine cycle. The spark plug firing that occurs during the compression stroke is called a xe2x80x9cusefulxe2x80x9d firing and the firing that occurs during the exhaust stroke is called a xe2x80x9cwastexe2x80x9d firing. The secondary voltage required by a spark plug during a compression firing is much higher than the voltage required during an exhaust firing.
Most present day ignition voltage testers use multiple secondary probes which are connected to each spark plug wire. Each probe has an assigned polarity, either positive or-negative. The operator is required to know the firing voltage polarity of each spark plug wire for a give engine to be able to connect the probes to the correct spark plug wires. This significantly increases the time required for the test since the operator must first look up the firing voltage polarity for each engine tested. This also increases the level skill required to properly test the ignition voltage.
Another disadvantage of most present day ignition voltage testers is that they use expensive displays to display the peak ignition voltages graphically. These displays include a cathode ray tube (CRT) or an LCD graphic display, for example. Also, these displays require complex circuitry to obtain the entire secondary waveforms, measure the peak ignition voltages and drive the displays, which further increases the cost of the tester. The displayed waveforms require a skilled mechanic to read and properly interpret the data.
Yet another disadvantage of most present day testers is that the probes used to pick up the secondary voltages Lack sufficient isolation. A typical probe includes a metallic spring-loaded, alligator-type clamp. The metallic frame of the clamp itself is used as the pickup. A clamp of this type picks up stray signals from adjacent spark plug wires. The clamp must therefore be moved sufficiently far away from adjacent spark plug wires, if possible, to obtain a proper reading.
The hand-held ignition voltage tester of the present invention is inexpensive, provides an easy to read output and is simple to use in that it requires no knowledge of spark plug polarity and resists stray pick-ups from adjacent spark plug wires. The tester includes a housing, positive and negative power cables and a capacitive probe for capacitively coupling to a spark plug wire. The capacitive probe generates a voltage signal which is representative of the voltage on the spark plug wire. A plurality of voltage level indicators are mounted to the housing, with each indicator corresponding to a selected voltage level. A measurement circuit is coupled to the power cables and the capacitive probe for selectively activating the voltage level indicators in response to the voltage signal. The measurement circuit activates each voltage level indicator for which the voltage signal exceeds the corresponding selected voltage level.
In one embodiment, the tester further includes an absolute value circuit and a plurality of comparators. The absolute value circuit has an input coupled to the capacitive probe and has a magnitude output. Each comparator has a first input coupled to the magnitude output, a second input coupled to a respective reference voltage and an output coupled to a respective one of the plurality of voltage level indicators.
The voltage level indicators are preferably arranged on the housing in a line to form a bar graph. In one embodiment, the tester further includes a low voltage indicator at a low end of the bar graph which is activated to indicate a low voltage condition when the magnitude output is less than a selected minimum voltage level. An open circuit indicator is positioned at a high end of the bar graph and is operated by the measurement circuit to indicate an open circuit condition when the voltage signal exceeds a selected maximum voltage.
In a preferred embodiment, the capacitive probe includes a conductive probe handle and a clamping member for clamping to the spark plug wire. A first insulator is attached to the clamping member, which insulates the clamping member from the spark plug wire. A shielded cable extends from the housing and is attached to the probe handle. The shielded cable includes an inner conductor, a second insulator surrounding the inner conductor, a conductive shield surrounding the second insulator, and a third insulator surrounding the conductive shield. The probe handle is grounded to the conductive shield. The inner conductor extends along the clamping member between the clamping member and the first insulator to form a capacitive pick up. The conductive shield is stripped from the inner conductor along the clamping member only.